Which of the following is NOT a valid formula according to the NEC?

The reasoning behind identifying the option related to the maximum load as not a valid formula according to the National Electrical Code (NEC) is fundamental to understanding load calculations and safety factors.

In electrical design, it is standard to determine loads and ensure that the equipment can handle those loads safely and reliably. The formulations that involve multiplying by factors (like 0.8 or 1.25) are based on established NEC guidelines that incorporate safety margins into equipment ratings, ensuring that the systems are not overloaded under typical conditions. For example, continuous loads are commonly calculated using a multiplier that helps to ensure that the equipment can sustain operation continuously without exceeding ampacity limits.

In contrast, a formula suggesting that the maximum load may be set at twice the table value does not align with the code's principles. Generally, the NEC allows for safe operation based on established factors rather than extremes. A multiplier of 2.0 would imply operating equipment at a level that could potentially indicate an unsustainable or unsafe use beyond reasonable limits that the NEC intends to safeguard against.

Therefore, while other formulas ensure safety and reliability through proper calculations, the use of a multiplier that drastically exceeds standard practices is what renders that specific formula invalid according to NEC standards.